As described in U.S. Pat. No. Re. 31,938, reissued Jul. 9, 1985 to Roger Klukow and assigned to the present assignee, web offset printing and production requires precision bending of lithographic plates to assure precise alignment of the images on the plates and longevity of the plates. Klukow describes a precision machine tool to position accurately and bend replicate lithographic plates in precise alignment with each other. According to the bending fixture described by Klukow, bends in the lithographic plate are created by rolling the metal around anvils at edges of a supporting bed using eccentric camming members. These camming members are supported throughout their length by a plurality of bearings, which provide effective control of the bending operation. The disclosure of the Klukow patent is completely and specifically incorporated herein by reference.
Prepunching of the plates with a precision register punch before positioning on the Klukow plate bending fixture assures that the image registration will be exactly positioned when the plate is bent. The plate is inserted onto the bed of the fixture and rides on an air cushion generated through the bed. Limit switches allow positioning of the plate, which is then aligned with registration pins in the support bed for proper registration of the plate and image for forming of the bend. Positioning of the plate with the aid of the limit switches and registration pins assures perfect alignment of the image on the plate relative to the formed bends.
Once the plate has been properly aligned on the Klukow bending fixture, reversal of air pressure immobilizes the plate on the table with vacuum pressure. The registration pins are retracted and the plate ends are clamped to position the plate precisely for bend forming. The eccentric camming members are precisely aligned to the radius of the anvil around which the bend is to be formed and to the exact thickness of the lithographic plate to be bent. The Klukow bending fixture uses a plurality of bearings to support each eccentric camming member throughout its length. This arrangement assures straightness of the bend and parallelism of the bends on the opposing edges of the plate and alignment of the image to each bend. The upper bearings lower each camming member to the plate, clamping the plate to the table. Each camming member rolls the lithographic plate around the anvil to form the bend. This assures that the plate remains of uniform thickness throughout, including the areas of the formed bends, thus avoiding any mechanical stress.
After formation of the bend with the Klukow bending fixture, each camming member is counter rotated to its original position on the plate. The retractable anvil withdraws laterally from the bend edge, the upper bearings raise the eccentric camming member, and the plate is removed from the Klukow fixture. The formed plate has a uniform thickness over its entire area, including the length of the bend, with minimum distortion of the plate.
Despite the apparent advantages of the Klukow fixture, it can only prepare lithographic plates bent to the angle of the bending radius of the anvil edge. However, printing presses manufactured by different companies have lithographic plate receiving slots that extend into the printing press cylinder at angles different than those of other manufacturers. This requires the printer to either have multiple benders each with an anvil unique to the printing press on which the lithographic plate is to be mounted or to remove and r place the anvils each time a lithographic plate is to be bent for a different printing press. In the first case, the extra bender is an expensive investment and in the second case the removal and replacement of the anvils on a bender is a time consuming and, therefore, costly procedure. So having a single bender capable of preparing bends at multiple angles for easy and accurate attachment of the plates into the receiving slots of a printing cylinder would be desirable. It is also desirable to have a bender capable of easy and efficient adjustment of the bend angle because it is not unusual to have a press cylinder that requires one angle of bend on one edge of the plate and another on a second edge of the plate. And each of these bends may vary from one manufacturer to another. This requires that the angle of the leading edge of the plate bender be adjustable independently of the trailing edge.
It is also known that sharp end corners of the bend can sometimes bind or crimp during installation of the bent plate into the slots in the printing cylinder. This impedes proper and efficient positioning of the plate about the cylinder. Cutting off the sharp end corners of the lithographic plate before bending allows the plate to be more easily mounted onto the printing press cylinder. Corner cuts of this type are generally low tolerance and can be cut at any angle in one or more corners of the printing plate. These corner cuts also provide clearance for obstructions internal to the printing plate cylinder. Incorporating corner cutters into the plate bending operation in a single apparatus would reduce the time and number of equipment pieces needed to produce a printing plate ready to mount to the printing press cylinder. Before bending, other selected shapes may be cut either from the corners or from along the plate edge to be bent. These cut-out shapes configure the plate to match various alignment and locking features on the printing cylinder, typical cut-out shapes are shown in FIG. 9.
Until recently, such corner cutters have been part of the printing plate punching operation. The punching operation had been used to control registration of the image on the printing plate and to cut plate corners for precise mounting onto the printing press cylinder. With the introduction of computer control of plate imaging, the printing plate punching operation is no longer used, but corner cutting of the printing plate is still required. Corner cutting assures the lateral alignment of the printing plate on the printing cylinder of the printing press. Correct plate alignment throughout the entire printing press will speed start-up time considerably resulting in reduction of high start-up costs. These corner and edge cuts must be controlled to a high tolerance to assure correct alignment to the image applied to the printing plate. Depending on the design of the printing cylinder, one or more of the printing plate corners may need to be precisely cut. The corner cuts usually fit into or along side of a block or edge of the printing cylinder and can assume a variety of sizes and shapes, to align with specific printing cylinder, including triangular and rectangular shapes.
World-wide, there are many manufacturers of different printing presses, each requiring a particular size, shape, or angle of bend or of corner or edge cut to mount and hold the printing plate to the printing press cylinder. Printing plate manufacturers use a variety of different materials and processes to manufacture printing plates. Typically, plates may be made of metal, such as steel or aluminum, or of synthetic photographic plate materials. The bending and springback characteristics of these materials vary, and it is generally necessary to over-bend the plate, so that the proper desired angle bend is attained after allowed material spring back.
Accordingly, there is a need for an apparatus and a method to form lithographic plate bends of different chosen angles. A need also exists for an apparatus and a method for preparing lithographic plates that will incorporate removal of the lithographic plate's sharp end corners during bending of the lithographic plate.
A cut along a perimeter of the plate is meant to include a corner cut of one to four corners of the plate, as well as cut-out shapes along opposite edges of the plate, that is the edges to be bent in the bending operation.